1Execute-in-place for file mappings
2----------------------------------
34Motivation
5----------
6File mappings are performed by mapping page cache pages to userspace. In
7addition, read&write type file operations also transfer data from/to the page
8cache.
910For memory backed storage devices that use the block device interface, the page
11cache pages are in fact copies of the original storage. Various approaches
12exist to work around the need for an extra copy. The ramdisk driver for example
13does read the data into the page cache, keeps a reference, and discards the
14original data behind later on.
1516Execute-in-place solves this issue the other way around: instead of keeping
17data in the page cache, the need to have a page cache copy is eliminated
18completely. With execute-in-place, read&write type operations are performed
19directly from/to the memory backed storage device. For file mappings, the
20storage device itself is mapped directly into userspace.
2122This implementation was initially written for shared memory segments between
23different virtual machines on s390 hardware to allow multiple machines to
24share the same binaries and libraries.
2526Implementation
27--------------
28Execute-in-place is implemented in three steps: block device operation,
29address space operation, and file operations.
3031A block device operation named direct_access is used to retrieve a
32reference (pointer) to a block on-disk. The reference is supposed to be
33cpu-addressable, physical address and remain valid until the release operation
34is performed. A struct block_device reference is used to address the device,
35and a sector_t argument is used to identify the individual block. As an
36alternative, memory technology devices can be used for this.
3738The block device operation is optional, these block devices support it as of
39today:
40- dcssblk: s390 dcss block device driver
4142An address space operation named get_xip_mem is used to retrieve references
43to a page frame number and a kernel address. To obtain these values a reference
44to an address_space is provided. This function assigns values to the kmem and
45pfn parameters. The third argument indicates whether the function should allocate
46blocks if needed.
4748This address space operation is mutually exclusive with readpage&writepage that
49do page cache read/write operations.
50The following filesystems support it as of today:
51- ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
5253A set of file operations that do utilize get_xip_page can be found in
54mm/filemap_xip.c . The following file operation implementations are provided:
55- aio_read/aio_write
56- readv/writev
57- sendfile
5859The generic file operations do_sync_read/do_sync_write can be used to implement
60classic synchronous IO calls.
6162Shortcomings
63------------
64This implementation is limited to storage devices that are cpu addressable at
65all times (no highmem or such). It works well on rom/ram, but enhancements are
66needed to make it work with flash in read+write mode.
67Putting the Linux kernel and/or its modules on a xip filesystem does not mean
68they are not copied.